III-Nitride materials are semiconductor compounds that have relatively wide direct bandgaps and can have strong piezoelectric polarizations, which can enable high breakdown fields, high saturation velocities, and the creation of two-dimensional electron gases (2DEGs). As a result, III-Nitride materials are used in many microelectronic applications as field-effect transistors (FETs), high electron mobility transistors (HEMTs), and diodes.
In many commercial applications, large area, low cost, and readily available substrates are needed for the deposition and crystal growth of III-Nitride materials, thin films, and resulting device structures. Consequently, many III-Nitride materials are grown on non-III-Nitride substrates, using one of several different thin film deposition techniques. However, III-Nitride materials have a different lattice constant/parameter than most commonly used non-III-Nitride substrate materials. In some cases, this lattice parameter difference, or lattice mismatch, can be relatively large, and can lead to the formation of crystal defects in the III-Nitride material layers that may impair the performance of devices formed using the III-Nitride material layers. While the III-Nitride materials are being epitaxially deposited, the lattice parameter mismatch of the III-Nitride structure interlayers can also build stress within the III-Nitride-substrate composite structure that may cause macroscopic deformation of the composite structure at growth temperatures. The stress and consequential deformation may tend to build as the thickness of the III-Nitrides layers increase and may reach deformation limits that. if exceeded. could result in excessive warp, bow and plastic deformation or slip of the composite structure. If this occurs during growth of the III-Nitride, the resulting deformation may result in physical separation of the composite structure from the growth platform. This can lead to mechanical instability of the composite structure within the deposition chambers as well as loss of uniform heating across the composite structure resulting in thickness and compositional non-uniformity for additional III-Nitride interlayer growth. The impact is a detrimental loss to fabricated device yields across the wafer and less than optimal designed device performance.
The inherent lattice parameter mismatches, associated strain on the composite structure, and consequential deformation during growth thus presents limitations to the thickness that can be achieved for the growth of III-Nitride materials in conventional structures. Such limitations may prevent the realization of III-Nitride device structures requiring relatively thick material stacks to achieve performance specifications such as those required by high voltage breakdown HEMTs, for example.
Additionally, differences in the material lattice thermal coefficients of expansion amongst and between the III-Nitride materials and the substrate can result in the development of significant stress during cool down front relatively high growth temperatures to room temperature that can also lead to large macroscopic deformation in terms of wafer warp and bow, epi structure cracking, as well as substrate plastic deformation and dislocation propagation (e.g. slip) of the resulting material-substrate structure. Such deformation can be large enough so as to make semiconductor device fabrication impracticable.